With the latest Firefox 9.1+ (pre-release of 1.0) intagrates fully with Outlook 2003.

Pushing Ctrl+M will give you a new mail window form outlook (or whatever you are using) or if you go to tools, you will see "read mail" which will open your default email program.

Download

When you click on tools you should see the new email options in the drop down menu. This should prove very useful for those wanting to check their email within firefox.